1. Field of the Invention
The present invention relates generally to a new automated laser welding system configured to produce, for example, an improved welded work piece, such as an automotive body panel, and a system and method for the manufacture thereof that includes an improved laser welder and a visual weld inspection device. The invention also relates to a method for performing an automated quality control inspection of a laser weld.
2. Background
In the past, welded work pieces such as body panels for use in the automotive vehicle industry were made by stamping or drawing the panel from either a single blank of a ductile sheet metal material, including steel, or from a plurality of such blanks that were previously welded together. Either type of welded work piece or body panel usually required the addition of stiffeners and pads welded to sections of the panel to improve its structural rigidity. The added stiffeners and pads were also needed to increase the thickness of the work piece in predetermined locations so that various structural and fastening assemblies could be fastened and welded to the panel without damage during the fastening or welding process. The addition of the stiffeners and pads increased the weight of the work piece and also increased the total manufacturing time needed to fabricate the work piece. The work pieces were often formed, drawn, or stamped into a final shape to have a three-dimensional shape corresponding to the overall design of the automotive vehicle.
As a result of the number of manufacturers in the field, the automotive vehicle industry is very competitive with respect to, among other things, quality, raw material costs, and manufacturing times required to completely fabricate and assemble a vehicle. To remain competitive, manufacturers have continuously expended enormous resources to contain, if not reduce, material costs by reducing part weight, part count, and manufacturing time while maintaining the needed high degree of quality. A considerable amount of such resources have been directed to improving and automating routine tasks such as the fastening together of various work pieces and vehicle parts such as, for example, body panels for fenders, quarter panels, trunk lids, engine compartment hoods, vehicle doors, and other various components.
Previously, multi-part sheet metal blanks have been welded together into a single work piece before being stamped into a final shape. These blanks were prepared by a variety of fastening techniques including chemical, arc, and CO.sub.2 laser welding, riveting, bolting, cold forming, and similar methods. Of particular interest in recent years is the use of more efficient laser welding using CO.sub.2 lasers in automated, numerically controlled manufacturing processes. Such laser welding can be accomplished for joining together sheet metal blanks at a common seam by means of, for example, a lap weld, or a butt weld. Butt welds are often preferred because only a single seam needs to be welded in contrast to lap joint which usually require that two seams be welded.
Many problems have been associated with the use of CO.sub.2 lasers including the requirement that less than optimum welding speeds must be used because of the poor absorption by steel work pieces of the energy produced by the CO.sub.2 laser. Also, laser welded joints can be plagued with problems despite the use of an appropriate weld speed if a manufacturer does not carefully prepare the work pieces or is otherwise not attentive to the intricacies and pitfalls of laser welding processes. Problems are even more prevalent when the blanks to be welded together are of dissimilar thickness. Such problems include, for example, mismatch between the welded parts along the joint on at least one exterior surface, poor weld bead dimensions or hardness, cracks, poor weld bead continuity across the length of the weld, and pinholes formed in the weld bead. Many of these welding problems are difficult to avoid and even more difficult to detect. More often than not, detection of such problems can only be accomplished by a slow and tedious visual inspection. Further, some of these problems, such as cracks, weld spatter, and pinholes, can only be detected through destructive testing such as by tension and shear tests, micrographic cross-sectional analysis, etch and penetrant dye inspections, and formability testing to ensure the welded blanks of the work piece can be drawn or stamped without failure anywhere along the welded joint.
These problems are especially apparent when steel work pieces, such as welded components for an automotive body or door panel, are to be butt welded together for form a larger, single work piece or door panel blank that can be later stamped or drawn into a shaped panel ready for painting and attachment to the vehicle. In many cases such welds are straight line weldments that could be completed faster if an improved laser welding technique were available. Additionally, it would be desirable to have an automated manufacturing assembly line wherein multiple work pieces could be automatically introduced to the laser welding apparatus to minimize the risk of injuries to workers from reflected laser energy. Further, such welding manufacturing processes could be made more efficient if a technique existed to speed up the post-weld inspection process.
There have been attempts to develop a viable method for laser welding inspection. U.S. Pat. No. 5,607,605 discloses such a method, which utilizes a CCD (Charge Coupled Device) camera to capture an image of the plasma generated when a laser beam contacts an object to be welded. The image is then sent to an image processing device, which measures a selected particular feature of the plasma cloud. The measurement is further transferred to a distinction device, which compares the measurement with a reference value to determine if the laser welding condition, and thus the weld, is acceptable.
Electro-optical detection of laser welding conditions has also been employed as an inspection method. U.S. Pat. No. 5,272,312 recites a method for the inspection of a laser weld, wherein the area of the material in contact with the laser beam, referred to as the laser processing spot, is projected onto at least one photodetector such as photodiode which detects the amount of liquid material ejected from the weld pool during the welding process. The signal from the photodiode can be converted into an electrical signal, which may then be sent to a processing unit for determination of the size and location of voids or pores in the weld seam. In one embodiment, this reference discloses the detection of ultraviolet radiation present in the plasma cloud.
Laser welding generates particular signals which may be monitored to determine the quality of a weld. U.S. Pat. No. 5,681,490 discloses that sensors such as photodiodes, phototransistors, photo darlingtons, pyroelectric detectors, microphones, and infrared and thermal detectors can be positioned to monitor various stages of the welding process. Such sensors may be utilized to monitor light, sound, gas, smoke, temperature, etc. The signals generated by these sensors may then be analyzed by a computer to predict the weld quality.
None of the prior art, however, discloses an apparatus or method utilizing direct inspection of the weld bead to determine the quality of a laser weld. The prior art methods generally depend upon the use of unstable process indicators to ascertain the condition of the weld, often requiring the monitoring and analysis of a multitude of signals to reach a conclusion regarding weld quality.
The automotive industry is in need of a laser welded work piece that contains fewer parts, has an optimally minimized weight, and that is produced through the use of an automated, laser welding manufacturing process. The welded work piece produced in accordance with the present invention, and the system and method for its manufacture, overcomes the deficiencies of the presently known methods for automated laser welding and inspection of welded work pieces.